The Fe active sites in FeZSM-5 catalyst for selective oxidation of CH4 to CH3OH at room temperature

It was shown recently that iron complexes formed during the thermal treatme nt of FeZSM-5 zeolite perform single-turnover cycles of methane oxidation t o methanol at ambient conditions when nitrous oxide is used as a source of oxygen.(1) The long-living active intermediate is capable of transferring a ccepted O atom (called alpha -oxygen) into C-H bond of methane to produce m ethanol at 100% selectivity.(2) The present work is aimed to the identifica tion of iron active sites through a comparison of in situ Fe-57 Mossbauer s pectra of FeZSM-5 after various thermal treatments and reaction stages. It is established that vacuum activation at 900 degreesC accompanied by a mani fold increase of alpha -centers leads to the transformation of inactive Fe3 + to the active, coordinatively unsaturated Fe2+ states. After alpha -oxyge n loading, active Fe2+ states transform to a new Fe3+ states responsible fo r further methane oxidation. The latter reaction, as well as reaction with H-2(2), is not fully reversible: part of active Fe3+ transforms to other in active Fe3+ form. On the contrary, reaction of alpha -oxygen with CO leads to a complete restoration of the initial, vacuum activated Fe2+ states. On the base of joint Mossbauer and catalytic data, the structure and compositi on of iron active centers are suggested.